Plex Media Server breaks Dolby Atmos playback

[jeedewee]

Server Version#: 1.28.0.5999
Player Version#: 5.32.1

I have several mkv files containing video, eac3 audio streams with Dolby Atmos and several subtitle streams. I noticed that if I play these files on my Samsung TV on my Sonos Arc the audio format is shown as Dolby Multichannel PCM 5.1 and not Dolby Atmos.

I have checked the log file (attached) and found that due to the 30 stream limit of the Samsung TV, the Plex Media server does a transcode. So far so good but the server also decides to:

[MDE] Server decision, directPlay: false, directStreamVideo: true, directStreamAudio: false

Then it decides to a transcode of the audio also as shown earlier in the log file:

Aug 23, 2022 09:20:45.937 [0x7f7f26caa0] INFO - [Req#414] [Plex for Samsung] [jeedewee:ouo0mhvvet8fdh1yy8rcsulr]   audioStream: {
  "bitrate": 774,
  "bitrateMode": "cbr",
  "channels": 6,
  "codec": "aac",
  "default": true,
  "displayTitle": "English (EAC3 5.1)",
  "extendedDisplayTitle": "English (EAC3 5.1)",
  "id": "16465",
  "language": "English",
  "languageCode": "eng",
  "languageTag": "en",
  "requiredBandwidths": "768,768,768,768,768,768,768,768",
  "selected": true,
  "streamType": 2,
  "decision": "transcode",
  "location": "segments-audio"
}

I do not believe it needs to do that. The transcode should only remove the unneeded subtitle streams and NOT transcode the audio.

Then i used ffmpeg to remove the unneccessary subtitle streams myself first by using the following command:

ffmpeg -i input.mkv -map 0:v -map 0:a -map 0:s:m:language:dut -map 0:s:m:language:eng -c copy output.mkv

Once I do that, there is no 30 stream limit anymore, the file automatically direct-plays and the audio shows up as Dolby Atmos on the Sonos Arc.

In my opinion this is a bug: transcode of audio should only happen when there is an incompatible format. That is not the case here,

plexmediaserver.log (10.3 KB)

[SledgeNZ]

Nice diagnosis. Also have noticed dropping atmos when it shouldn’t so thanks for highlighting the cause.

Would be good to get a response from Plex on this.

[sLumpy]

If memory serves me right, I believe that this is on purpose. If (e)AC3 is not Direct Played, it cannot be played properly by the TV, even when Direct Streamed. It causes audio to skip/cut out every now and then. I would need to dig further to reconfirm. I’ve logged an internal issue to take a look again but I don’t think this is behaviour we can change.

[jeedewee]

Thanks @SledgeNZ and @sLumpy for your response.

@sLumpy if I read your response correctly you are stating that EAC3 should always be direct-played. I think that is what it doesn’t do as outlined in my post. Plex media server transcodes the audio to multichannel AAC.

Anyway, I look forward to hearing what the outcome is of the internal issue you logged.

Thanks,
JeeDeWee